Voltage stability is a critical consideration in improving the security and reliability, for example, of power systems of public utilities and those power systems used in industry. The Static Compensator (STATCOM), a popular device for reactive power control based on gate turn-off (GTO) thyristors, has attracted much interest in the last decade for improving power system stability; see, for example, Fangxing Li (the present inventor) et al, 2006, A preliminary analysis of the economics of using distributed energy, as a source of reactive power supply. Oak Ridge National Laboratory (ORNL) Technical Report (ORNL/TM-2006/014), Oak Ridge, Tenn., April 2006. Various control methods have been proposed for STATCOM control. Amit Jain et al, in Voltage regulation with STATCOMs: Modeling; control and results, IEEE Tram. Power Del, vol. 21, no. 2, pp. 726-735, April 2006 among others introduced nonlinear optimal control. Clark Hochgraf et al., in STATCOM Controls for Operation With Unbalanced Voltage. IEEE Trans. Power Del., vol. 13, no. 2, pp. 538-544, April 1998, presented a synchronous frame voltage regulator to control system voltage by using separate, regulation loops for positive and negative sequence components of the voltage. Gerardo E. Valderrama et al., in Reactive Power and Unbalance Compensation Using STATCOM with Dissipativity-Based Control. IEEE Trans. Control Sysf. Technol, vol. 19, no. 5, pp. 598-608, September 2001, proposed proportional integral (PI) structures with feed forward to improve STATCOM performance. H. F. Wang, for example, in Phillips-Heffron model of power systems installed with STATCOM and applications, IEE Proc.-Gener. Transmi. Distib., vol. 146, no 5, pp. 521-527, September 1999 introduced a STATCOM damping controller to offset the negatiye; damping effect and enhance system oscillation stability. These non-patent literature articles mainly focus on the control structure design rather than exploring how to set PI control gains.
In many STATCOM systems, the control logic is implemented with PI controllers. The control parameters or gains play a key factor in performance. Presently, few studies have been, carried put on the control parameter settings. In many practices, the, PI controller gains are designed in a case-by-case study or trial-and-errpr.approach with tradeoffs ifi performance and efficiency. Generally speaking, it is hot feasible for utility engineers to perform extensive trial-and-error studies to find suitable parameters for each new STATCOM connection. Further, even if the control gains have been tuned to fit reasonable projected scenarios, performance may disappoint when a considefable change of the system conditions occurs, such as, for example, when a transmission line upgrade cuts layer replacing an old transmission line. The response can be particularly worse if the transmission topology change is due to an unexpected contingency. Thus, the STATCOM control system may not perform well when it is needed most.
A few, but limited, previous works in the non-patent-literature discuss the STATCOM PI controller gains in order to better enhance voltage stability and to avoid time-consuming tuning. Pranesh Rao et al. , in STATCOM Control for Power System. Voltage Control Applications. IEEE, Trans. Power Del., vol. 15, no, 4, pp. 1311-1317, October 2000, among others, propose a linear optimal,control based on linear quadratic regular (LQR) control. Since the gains in such LQR control depend on a designer's choice of factors of a weighting matrix, the optimal parameters depend on me designer's experience.
An Luo et al. in Fuzzy-PI-Based Direct-Output-Voltage Control Strategy for the STATCOM Used in Utility Distribution Systems. IEEE Trans. Ind. Electron., vol. 56, no. 1, pp. 2401-2411, July 2009 among others propose a fuzzy PI control method to tune PI controller gains. However, the fuzzy control method essentially gives an approximate recommendation. It is still up to the designer to choose the fixed deterministic gains. Therefore, again, the designer's experience may affect the final results. Further, a tradeoff of performance and the variety of operation conditions still has to be made during the designer's decision-making process.
A motivation in the art may be to design a control method that can ensure a quick and desirable response when the system operation condition varies in an expected or even an unexpected manner. The change of the external conditions; should not have ;a significant negative impact on the performance. Here the negative impact may refer to slower response, overshoot, or even instability of a power system. Based on this; fundamental motivation, an adaptive control approach for STATCOM to enhance voltage stability is an object of the present invention.
Given the foregoing, what is needed is a method and apparatus for adaptively controlling a static compensator (STATCOM) for a power system to enhance voltage stability whenever a negative impact on the power system performance occurs.